Electrically charged targets

ABSTRACT

Electrically chargeable laser targets and method for forming such charged targets in order to improve their guidance along a predetermined desired trajectory. This is accomplished by the incorporation of a small amount of an additive to the target material which will increase the electrical conductivity thereof, and thereby enhance the charge placed upon the target material for guidance thereof by electrostatic or magnetic steering mechanisms, without adversely affecting the target when illuminated by laser energy.

The invention described herein was made in the course of, or under,Contract No. W-7405-Eng48, with the United States Energy Research andDevelopment Administration.

This is a division of application Ser. No. 698,557, filed June 22, 1976,now U.S. Pat. No. 4,138,609, issued Feb. 6, 1979.

BACKGROUND OF THE INVENTION

This invention relates to laser targets, and more particularly toelectrically chargeable laser targets and method for forming suchcharged targets.

In certain magnetic confinement CTR systems, energetic neutral beams areinjected and trapped by interaction with a target plasma. These targetplasmas are generated by introducing into the confinement region a solidtarget or pellet of appropriate material which is vaporized and ionizedby means of laser beams. This requires the capability to accuratelyguide the target to the desired focal spot. The standard way ofaccomplishing this task is to place an electrical charge on the target,whereby the trajectory can be influenced by standard electrostaticand/or magnetic means. Articles by C. D. Hendricks entitled "ChargedDroplet Experiments", Journal of Colloid Science, vol. 17, No. 3, March1962; and "Micron and Submicron Particle Production", IEEE TransactionsOn Industry Applications, Vol. 1A-10, No. 4, July/August 1974, pp.508-510, exemplify prior known techniques in the forming of chargedparticles or targets. Also, the production of pellets using a fluid jetprocess is taught by J. M. Schneider et al., Rev. Sci. Inst. 35, 1349,1964.

The ability to guide a target or pellet is directly related to themagnitude of the charge carried by a target. Accordingly, a target musthave a relatively high electrical charge so as to be effectively guidedor directed by guidance apparatus, such as that exemplified in U.S. Pat.No. 3,582,958 issued June 1, 1971 in the name of C. D. Hendricks, Jr.;and an article by J. R. Adam et al. entitled, "The Collision,Coalescence, and Disruption of Water Droplets", Journal of AppliedPhysics, Vol. 39, No. 11, 5173-5180, October, 1968. Thus, a need existsfor a method of imparting a relatively high electrical charge upon atarget and to do so without application of undue voltages which mightproduce breakdown of the atmosphere in the target-generating chamber,since the charge on the targets which can be achieved by prior knownmeans is below the value required by electronic guidance systems foreffective guidance off the targets.

SUMMARY OF THE INVENTION

The present invention provides a solution to the limited charge problemon laser targets for effective guidance. In general, the inventioninvolves an electrically chargeable target formed by incorporating intothe target material an additive which will raise the electricalconductivity of the target material. For example, in magneticconfinement CTR applications wherein the target material is ammonia, theaddition of dilute hydrochloric acid (HCl) increases the conductivityratio of the ammonia by two orders of magnitude. Similarly, theinvention is applicable to laser fusion type targets wherein the targettrajectory is controlled by electrostatic or magnetic guidancemechanism. Thus, the present invention provides a way to impartrelatively high electrical charges upon a pellet or target, and to do sowithout application of undue voltages which might produce breakdown ofthe atmosphere in the target-generating chamber.

Therefore, it is an object of the invention to provide an electricallychargeable laser target.

A further object of the invention is to provide a laser target ofimproved chargeability for improving guidance thereof along apredetermined trajectory.

Another object of the invention is to provide a method for formingimproved electrically charged laser targets.

Another object of the invention is to provide a method for formingelectrically charged laser target by incorporating into the targetmaterial an electrically conductive additive such that relatively highelectrical charges can be imparted to the target.

Other objects of the invention will become readily apparent from thefollowing description and accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

The single FIGURE schematically illustrates a pellet or target generatorfor producing the electrically chargeable targets of the invention.

DESCRIPTION OF THE INVENTION

The invention overcomes the limited charge problem of laser targetswhereby such targets can be effectively guided along a desiredtrajectory via an electrostatic or magnetic guidance system. Broadly theinvention involves an electrically chargeable laser target and method offorming such charged targets. More specifically the target of thisinvention is formed by the incorporation into the target material of anadditive which increases the electrical conductivity of the target,thereby enhancing the charge placed upon the target without applicationof undue voltages which might produce breakdown of the atmosphere in thetarget-generating chamber.

The mechanism for forming prior known laser targets for a magneticconfinement CTR system, for example, is shown in the drawing, generallyindicated at 10, and comprises a stream of liquid 11 with a desiredperiodic restriction in diameter formed from a source of liquid underpressure indicated by arrow 12 directed through a housing or channel 13defining a liquid channel terminating in an orifice 14 forming a 10⁻² cmdiameter opening, and having a piezoelectric means 15, such as acrystal, positioned about channel 13 for impressing upon the liquid apressure wave of desired amplitude and frequency, such as 15 KHz. Theliquid 12 may be ammonia, for example. The liquid stream 11 emitted fromorifice 14, which is at this point still substantially continuous,passes through apertures in a pair of spaced electrically conductiveplates 16 and 17 having voltage pulses of about 1 Kv at 50 μsec, forexample, impressed thereacross, wherby the liquid segment between theplates is polarized, i.e., it becomes positively and negatively chargedat the respective ends of the liquid segment near the plates 16 and 17.When the stream 11 breaks up into drops, the drops formed from theserespective end sections retain the induced charges. For example, plates16 and 17 may be spaced 1.0 cm from each other, and with the velocity ofstream 11 being about 13 m/sec, about 1drop having a diameter of about2×10⁻² cm is formed by each liquid segment between the plates during a50 μsec pulse with an overall production of an arbitrary number, up tocrystal frequency, of drops per min. The thus formed drops becometargets or pellets 18 which pass between a pair of spaced deflectionplates 19 and 20 having for example, a voltage of 1 Kv impressedthereacross such that a desired number of targets 18 at selected spacedintervals are deflected from the stream of such targets past a barrieror baffle 21 and directed through an aperture 22 in wall 23 into amagnetic confinement chamber 24 or outer point of use. The magnitude ofthe charge placed on the drops formed from the end sections of eachliquid segment is limited by the magnitude of the electric field ofplates 16 and 17, which in turn is limited by the breakdown threshold ofthe background gas between the plates. As shown the target generatingmechanism 10, deflection plates 19 and 20, and barrier 21 are locatedwithin an evacuated chamber 25 formed by a housing 26, said chamber 25having a pressure of 45 Torr, for example, while confinement chamber 24has for example a pressure of <1 Torr, with wall 23 being common tochamber 25 and chamber 24.

As pointed out above, the problem is that the charge on the targets 18,as they pass through plates 16 and 17, which can be achieved byconventional means is below the value required for effective guidancethereof by electronic (electrostatic or magnetic) target guidancesystems.

The present invention provides a solution to this limited charge problemwithout modification of the target generating apparatus such asillustrated in the drawing. In general, the invention is to incorporatein the liquid material 12 an additive which will, with smallconcentrations, raise the electrical conductivity of the liquid medium.In the case of ammonia as the liquid 12, as exemplified above, theadditive comprises dilute (˜0.4%) of 37% hydrochloric acid (HCl) inwater, introduced into the anhydrous ammonia stream, as indicated byarrow 27, at a rate of less than 3% per volume. This increases theconductivity ratio of the ammonia by two orders of magnitude, andproduces a proportional increase in he charge density per drop or target18.

It has thus been seen that the present invention provides anelectrically chargeable laser target and method of forming same whichhas application in both the magnetic confinement CTR and laser fusionCTR systems, and which overcomes the limited charge problem associatedwith such targets which are guided along a predetermined trajectory byelectrostatic, magnetic, or similar guidance mechanism.

While particular materials, parameters, and apparatus have beendescribed and/or illustrated modifications and changes will becomeapparent to those skilled in the art, and it is intended to cover in theappended claims all such modifications and changes that come within thespirit and scope of the invention.

What we claim is:
 1. An electrically charged laser target capable ofbeing at least vaporized and ionized by laser energy for producing aplasma comprising: a quantity of liquid material composed essentially ofanhydrous ammonia and an additive means for significantly increasing theelectrical conductivity of said anhydrous ammonia and for enhancing theguidance capability of the target material by electronic guidancesystems said additive means being composed of dilute HCl.
 2. The targetdefined in claim 1, wherein said additive comprises about 0.4% of 37%HCl in water mixed with ammonia at a rate of less than 3% per volume. 3.The target defined in claim 1 formed by a process comprising the stepsof: mixing about 0.4% HCl in water with anhydrous ammonia at a rate ofless than 3% per volume, forming drops of the mixture, and inducing anelectrical charge on the thus formed drops producing electricallycharged targets.